1. Field of the Invention:
The present invention relates to an improved process for the recovery of uranium values contained in phosphoric acid, and, more especially, to the recovery of uranium from wet-process phosphoric acid produced by the acidulation of phosphate rock, via liquid-liquid extraction.
2. Description of the Prior Art:
It is well known to this art to recover uranium from aqueous solutions which contain the uranium at low levels of concentration, by separating it from the other constituents (which themselves can be diverted to valuable end uses in many instances) of the treated ores, by means of a combination of liquid-liquid extractions and chemical treatments, the goal of which is to isolate the uranium and recover it in the form of the oxide U.sub.3 O.sub.8 in a high state of purity, which can be used as a nuclear fuel source. These uranium recovery processes are applicable to ores such as phosphate rock which also yield phosphoric acid, or ores of various other origins, having a greater or lesser content of uranium, in most cases the uranium being present in oxide form. Such processes generally comprise treating or acidulating the ore by means of a strong concentrated acid, such as sulfuric, phosphoric, hydrochloric or nitric acid, to provide an aqueous solution which contains uranyl ions in a highly dilute state, in conjunction with other contaminating ions from which the uranium is recovered.
A typical example of a process for the recovery of uranium from wet-process phosphoric acids is described in U.S. Pat. No. 3,711,591 to Hurst and Crouse. This process features two successive extraction cycles. In the first cycle, the uranium of valency +6 in the impure phosphoric acid is extracted by means of a first organic extractant comprising an inert diluent, a primary extractant, i.e., di(2-ethylhexyl)phosphoric acid (HDEHP, DEPA or D2EPHA), and a synergistic extractant therefor, i.e., trioctylphosphine oxide (TOPO), at selected concentrations in the diluent. After separation of the phases, the impure acid which is exhausted in respect of uranium is returned to the phosphoric concentration unit while the organic phase which is charged with hexavalent uranium (VI) is reductively stripped with an aqueous solution of phosphoric acid, at a moderate flow rate, said aqueous solution containing a reducing agent based upon metallic iron or a ferrous iron (II) salt, thereby re-extracting tetravalent uranium (IV) in the aqueous phase. The organic phase which is depleted of its uranium values, after separation of the phases, is recycled to the extraction step of the impure acid, typically oxidized wet-process acid.
In the second cycle of this process, the aqueous solution of phosphoric acid, which has a high content of uranium and iron, produced as described above, is subjected to a second continuous extraction, after re-oxidation of the uranium to a valency of +6, by means of a second organic extractant of the same nature as the first extractant, to thereby produce an organic phase which is rich in uranium (VI) and an exhausted aqueous phase which is recycled to the re-extraction operation of the first cycle or which is diverted to the phosphoric concentration unit. The aforesaid organic phase is washed with water and then treated to recover therefrom the uranium by precipitation, in the form of a mixed carbonate of uranium and ammonium, i.e., ammonium uranyl tricarbonate (AUT). The resulting uranium-depleted organic phase is recycled. The AUT is thence conveniently calcined into U.sub.3 O.sub.8.
Industrial use of this process requires that, in order to achieve effective re-extraction of the uranium from the first extractant, the concentration of iron in the aqueous solution of phosphoric acid must be relatively high, on the order of 15 to 30 g/liter of aqueous solution. Under these conditions, and in spite of a very low degree of extraction in respect of the iron by the organic phase in the second cycle, the uranium produced contains substantial amounts of iron which can be up to 4 to 5%. There exists, therefore, serious need for an improved process which enables recovery of uranium which is substantially free from iron under economically viable conditions.
See also, generally, U.S. Pat. Nos. 2,859,092, 3,737,513, 3,835,214 and 4,105,741, and French Pat. Nos. 2,070,722, 2,241,620, 2,249,841 and 2,249,961.